I've encountered a similar bug. Me and a friend were on multiplayer. I was on Android and he was on IOS. The world we were playing on was my android world. We had hardmode gear and we were doing another (This one, we were doing one before this one) world to get all the new goodies of the 1.2 update. We kept trying to spawn the wall of flesh, throwing voodoo dolls into lava. He never spawned doing that. Then we tried to spawn him by killing a voodoo demon over the lava pit we were using. We did it twice. Both didn't spawn him.